Fix TestEnv for subpackages: precompile only the new test Project

[NHDaly]

Fixes #89.

Woohoo! :) It seems to work now! :)

@davidanthoff: This should also make testing subpackages faster in VSCode as well, not just for ReTestItems.

---

I'm happy to clone 1.9 into a 1.10 directory, and make this change there, but it doesn't seem to hurt anything on older versions so it seems like we can keep things as-is?

[NHDaly]

FYI the julia 1.0 - macos job failed with the following:

==> macos OS detected
https://uploader.codecov.io/latest/macos/codecov.SHA256SUM
==> SHASUM file signed by key id 806bb28aed[7](https://github.com/JuliaTesting/TestEnv.jl/actions/runs/11391513338/job/31695476316?pr=90#step:8:8)79[8](https://github.com/JuliaTesting/TestEnv.jl/actions/runs/11391513338/job/31695476316?pr=90#step:8:9)69
==> Uploader SHASUM verified (75086ee436f6f74c4e8f65448eaa3[9](https://github.com/JuliaTesting/TestEnv.jl/actions/runs/11391513338/job/31695476316?pr=90#step:8:10)9e38bc93bb27a53d057b24b275b9f78c89  codecov)
==> Running version latest
==> Running version v0.8.0
/Users/runner/work/_actions/codecov/codecov-action/v3/dist/codecov -n  -Q github-action-3.1.6 -f lcov.info
Error: spawn Unknown system error -86
    at ChildProcess.spawn (node:internal/child_process:421:[11](https://github.com/JuliaTesting/TestEnv.jl/actions/runs/11391513338/job/31695476316?pr=90#step:8:12))
    at Object.spawn (node:child_process:761:9)
    at ToolRunner.<anonymous> (/Users/runner/work/_actions/codecov/codecov-action/v3/webpack:/codecov-action/node_modules/@actions/exec/lib/toolrunner.js:4[13](https://github.com/JuliaTesting/TestEnv.jl/actions/runs/11391513338/job/31695476316?pr=90#step:8:14):1)

definitely seems like a flakey unrelated failure

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 10.46%. Comparing base (3d198c7) to head (c30e22d).

Files with missing lines	Patch %	Lines
src/julia-1.11/activate_set.jl	0.00%	1 Missing ⚠️
src/julia-1.11/common.jl	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (3d198c7) and HEAD (c30e22d). Click for more details.

HEAD has 6 uploads less than BASE

Flag	BASE (3d198c7)	HEAD (c30e22d)
	7	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #90       +/-   ##
===========================================
- Coverage   32.10%   10.46%   -21.65%     
===========================================
  Files          28       28               
  Lines         707      707               
===========================================
- Hits          227       74      -153     
- Misses        480      633      +153     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[NHDaly]

🤔 the 1.9 / 1.10 test failures seem real:

activate test/Project: Error During Test at /Users/runner/work/TestEnv.jl/TestEnv.jl/test/activate_set.jl:29
  Got exception outside of a @test
  The following 1 direct dependency failed to precompile:
  
  NetCDF [30363a11-5582-574a-97bb-aa9a979735b9]
  
  Failed to precompile NetCDF [30363a11-5582-574a-97bb-aa9a979735b9] to "/Users/runner/.julia/compiled/v1.9/NetCDF/jl_KVLanS".
  ERROR: LoadError: InitError: could not load library "/Users/runner/.julia/artifacts/0b92c3ca1de2e9dc46a381553278edd25af54980/lib/libnetcdf.18.dylib"
  dlopen(/Users/runner/.julia/artifacts/0b92c3ca1de2e9dc46a381553278edd25af54980/lib/libnetcdf.18.dylib, 0x0001): Library not loaded: '@rpath/libmbedtls.13.dylib'
    Referenced from: '/Users/runner/.julia/artifacts/0b92c3ca1de2e9dc46a381553278edd25af54980/lib/libnetcdf.18.dylib'
    Reason: tried: '/Users/runner/.julia/artifacts/0b92c3ca1de2e9dc46a381553278edd25af54980/lib/./libmbedtls.13.dylib' (no such file), '/Users/runner/.julia/artifacts/0b92c3ca1de2e9dc46a381553278edd25af54980/lib/./libmbedtls.13.dylib' (no such file), '/Users/runner/hostedtoolcache/julia/1.9.4/x64/lib/julia/libmbedtls.13.dylib' (no such file), '/Users/runner/hostedtoolcache/julia/1.9.4/x64/lib/julia/../libmbedtls.13.dylib' (no such file), '/Users/runner/hostedtoolcache/julia/1.9.4/x64/bin/../lib/libmbedtls.13.dylib' (no such file), '/usr/local/lib/libmbedtls.13.dylib' (no such file), '/usr/lib/libmbedtls.13.dylib' (no such file)

but weirdly the tests all pass for me locally, on julia 1.9, 1.10, and 1.11 on both macos and linux... 🤔 any ideas?

JULIA_PKG_SERVER_REGISTRY_PREFERENCE=eager julia +1.10 --proj --startup=no -e 'using Pkg; Pkg.test()'
...
Test Summary: | Pass  Total  Time
TestEnv.jl    |   16     16  6.1s
     Testing TestEnv tests passed

[NHDaly]

Gonna try checking if CI is just broken due to something transient in the Pkg registry or something like that, by opening this PR: #91.